All cattle greater than or equal to 24 months-old in 6 M. paratuberculosis-infected Wisconsin dairy herds were sampled every 6 months for a period of 2.5 years (5 times total). Serum antibodies were measured by ELISA using the M. paratuberculosis Antibody Test Kit (IDEXX Laboratories, Inc.). The gamma interferon (IFN) response of peripheral blood leukocytes stimulated with M. bovis or M. avium PPD was determined using the M. paratuberculosis Gamma-Interferon Test Kit (IDEXX Laboratories, Inc.). Fecal samples were decontaminated with 1.0% HPC and M. paratuberculosis growth was detected using a modified BACTEC radiometric culture system. Whenever possible, ileum and regional lymph node samples from animals that were culled from the herds were collected at slaughter and processed for isolation of M. paratuberculosis and for histopathology. A total of 847 animals were tested. Of these, 373 were present at the first herd test and sampled at least twice more. The case definition for paratuberculosis was isolation of M. paratuberculosisfrom a fecal or tissue sample; 106 animals present in herds at the first test fit this case definition. The pattern of conversion to positive test status was variable among cattle. The host response to mycobacteria paradigm is that a cell-mediated immune response, eg. IFN, will precede a humoral immune response. For 34% of cattle, the IFN test became positive before the ELISA. For 34% of cattle, the IFN became positive before fecal culture. For 23% of cattle the ELISA was positive before the animal became fecal culture positive. For 40% of cattle studied, neither the IFN nor the ELISA became positive before the animal became fecal culture positive and was culled from the herd. Other patterns of conversion to test positivity were also examined. The variability observed between animals suggests that the response of cattle to M. paratuberculosis infection is rather unpredictable. Some factors that may govern the pattern of responses to infection include age of exposure, dose of M. paratuberculosis, and host genetics.